1. Field of the Invention
This invention relates generally to a pressure regulator and, more particularly, to a pressure regulator that includes multiple valve stages to increase the turn-down ratio of the regulator.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include about two hundred or more cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
Flow control pressure regulators are employed in fuel cell systems at various locations to provide a desired gas flow rate. For example, pressure regulators are typically employed at the anode side of the fuel cell stack to provide a pressure reduction of the hydrogen gas flow from a hydrogen pressure storage tank and at an anode inlet to the stack. At the output of the hydrogen pressure tank, the pressure regulator may be required to reduce the pressure from 30-700 bar tank pressure to 4-9 bar line pressure. At the input to the anode side of the fuel cell stack, the pressure reduction may be from 4-9 bar line pressure to 1-2 bar stack pressure. In both of these applications, the hydrogen flow rate may vary between 0.02 and 2.0 g/s. These parameters provide a regulator turn-down ratio, i.e., range of operation, of about 1:100. However, known pressure regulators are generally designed for turn-down ratios in the range of 1:10-1:20, and typically require a relatively constant inlet pressure. These types of pressure regulators are not suitable for fuel cell system applications because of the accurate pressure regulation of low flow rates, and the tight flow control necessary for the anode input.
Pressure regulation in a flow control pressure regulator is usually provided electronically using proportional valves or injectors. Alternately, a passive control can be used where the pressure regulator is responsive to a reference pressure, such as the cathode input pressure. FIG. 1 is a cross-sectional view of a pressure regulator 10 of this type. The pressure regulator 10 includes a regulator body 12 defining the various ports, chambers, orifices, etc. in the regulator 10. The gas flow is introduced at an inlet port 14 and exits the regulator 10 at an outlet port 16. The gas from the inlet port 14 flows through a valve chamber 18, then through an orifice 20 and into a chamber 22 in fluid communication with the outlet port 16.
The flow of the gas from the inlet port 14 to the outlet port 16 is controlled by a valve 28 positioned within the valve chamber 18. The valve 28 includes a valve head 30, a valve body 32 and a valve spring 34 wound around the valve body 32. The valve spring 34 applies a bias against the valve head 30 as set by a positioning element 26 threaded into the valve chamber 18. The valve head 30 seats against a tapered valve seat 36 to close the orifice 20. In other valve designs, the valve seat 36 may not be tapered. A shaft 38 is rigidly coupled to the head 30 and a cylindrical member 40 that is part of a membrane assembly 42. The membrane assembly 42 includes a support structure 44 having a central bore 46 in which the cylindrical member 40 is rigidly mounted. The membrane assembly 42 also includes a pair of membranes 48 and 50 mounted to opposing sides of the support structure 44 and the valve body 12, as shown, where the chamber 22 is below the membrane 50. In a hydrogen environment, it is necessary that the hydrogen be tightly contained within the proper chambers and flow channels. For this reason, the membrane assembly 42 includes the dual membranes 48 and 50. If the membrane 50 leaks hydrogen, then this leak can be detected at port 66 before it then leaks through the membrane 48 and into the air side of the regulator 10.
A spring 54 is positioned within a spring chamber 56 in contact with the support structure 44 at one end and a positioning screw 58 at an opposite end. A reference port 60 is in fluid communication with the chamber 56. The reference port 60 is coupled to a reference pressure for the particular application. The reference pressure may be ambient for the tank pressure regulator, or may be coupled to the cathode input pressure for the anode input pressure regulator. The bias of the springs 34 and 54 are calibrated by adjusting the screw 58 so that the pressure applied to the membrane assembly 42 against the bias of the springs 34 and 54 in combination with the reference pressure positions the membrane assembly 42 at a desired location relative to the chamber 22.
When a greater hydrogen flow rate demand is desired, such as for an increased load from a fuel cell stack (not shown), the stack will draw more hydrogen fuel, which will decrease the pressure at the outlet port 16. This decrease in pressure is transferred to the chamber 22, which causes the membrane assembly 42 to move downward with the bias of the spring 54 and against the bias of the spring 34. As the valve-body 32 moves down into a bore 68 in the positioning element 26, the head 30 moves farther away the valve seat 36, where the configuration of the valve seat 36 causes more hydrogen to flow from the inlet port 14 providing the increased flow rate. As the hydrogen demand decreases, the pressure at the outlet port 16 will increase, and the membrane assembly 42 will move up against the bias of the spring 54 and move the head 30 closer to the valve seat 36 to reduce the flow rate in the same manner.
Because the size of the orifice 20 at the valve seat 26 is fixed, the flow rate between a fully closed position and a fully opened position of the valve 28 is also fixed. It is for this reason that the pressure regulator 10 has a low turn-down ratio. When the pressure regulator is designed, the size of the orifice 20 is selected to provide the maximum flow that will be demanded. However, this provides less flow sensitivity at low flow rates because as the valve head 30 moves away from the valve seat 36, the size of the orifice 20 would be too large for low flow control, possibly causing the valve 28 to oscillate.